Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Diode based light sources are used everyday in a variety of applications right from blinking lights in indicators to modern communication systems, lighting applications, optical storage media etc. Solid state lighting is the holy grail for inexpensive and efficient lighting systems in future. The approach is to produce a commercially viable, low cost, and a successful high efficient lighting device that can act as a catalyst to reduce global warming (low carbon footprint). Earlier approaches relied on III-V based semiconductor materials, which are expensive and have issues of stability, and manufacturing process.
ZnO has always been considered as one of the most promising material candidates for ultraviolet light emitting diodes and laser diodes because of its direct-wide band gap (3.37 eV) and relatively large exciton binding energy (60 meV), compared with that of GaN (25 meV) and ZnSe (22 meV), at room temperature. The successful demonstration of room temperature electroluminescence from ZnO based homo- and hetero-junctions has proved that the material is ideal for the next generation short wavelength optoelectronic devices. Recent studies focus on the growth of p-type ZnO, which has the problems of doping asymmetry and the stability of p-type conductivity. All possible p-type dopants, such as, group-V elements [N, P, As, Sb], group-I and IB elements [Li, Na, K, Ag, Cu], have been tried to realize p-type conduction and ultraviolet (UV)-electroluminescence in ZnO. Nitrogen substitution at the oxygen site in ZnO appears to be promising for p-type conduction and many groups have reported room temperature electroluminescence from N doped ZnO based homojunctions. Dopants such as P, As and Sb seem to be good candidates for p-type conduction on ZnO. Among group I elements, theoretically Li possesses shallow accepter levels. However, no electroluminescence is achieved from Li substituted (LiZn) ZnO based homojunctions. In contrary to theoretical predictions, it was observed experimentally that Li doping in ZnO increases the resistivity drastically to several orders and makes it insulating.